Onlookers at Sotheby's in London were stunned when, just moments after selling for more than £1m, the Girl With Balloon painting shredded in front of them, coming out of the bottom of the frame in strips.

Image:The painting had just sold for more than £1m

Now, Banksy has revealed how he did it.

A video on his official Instagram page shows the device being installed, and says: "A few years ago I secretly built a shredder into a painting

"In case it was ever put up for auction "

It does not show how the shredder was activated, but it is thought to have been achieved through a remotely activated mechanism within the frame.

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One of Banksy's best-known images, Girl With Balloon was one of the star attractions at Sotheby's Contemporary Art Evening Sale.

The artwork was signed and dedicated and the vendor acquired it from the artist in 2006, the auction house said.